Rule Definition
A Service Contract basically describes the operations a service exposes to another party (in other words a client). It's recommended to apply the ServiceContract attribute to an interface, although it can be applied to a class as well. Operation Contract is applied to a method to indicate that the method implements a service operation as part of a service contract. Thus to have a service contract without any meaningful operation contract(method) to do a task indicates missing logic.

Remediation
Always define a service operation method and annotate it with Operation Contract

Violation Code Sample

[ServiceContract]  
interface ISimpleService  
{  
     string SimpleOperation();  //Violation
}

Fixed Code Sample

[ServiceContract]  
interface ISimpleService  
{  
     [OperationContract]  //Violation Fixed
     string SimpleOperation();  
}

https://docs.microsoft.com/en-us/dotnet/api/system.servicemodel.operationcontractattribute?view=dotnet-plat-ext-3.1